Traditional Chinese Simplified Chinese

  • Occurrence of Multiple Tropical Cyclones

  • Wednesday, 21st September 2016

Tropical cyclone development over the western North Pacific and the South China Sea was rather active in August 2016 with a number of tropical cyclones forming during the month. At one time on the morning of 20 August, there were four cyclones (Dianmu, Mindulle, Lionrock and Kompasu) in co-existence over the basin.

According to the Observatory's records since 1960, cases with three tropical cyclones co-existing over the western North Pacific and the South China Sea can be found nearly every year. Cases of four tropical cyclones occurring at the same time were less frequent with only four cases since 2000 as detailed in the table below:

Table 1

The highest number of tropical cyclones in co-existence in this basin is five. It happened twice, in 1960 (Figure 1) and 1985 (Figure 2). On the morning of 23 August 1960, Elaine was over the sea areas east of Taiwan, Carmen to the west of the Korean Peninsula, and Della, Faye and Bess over the western North Pacific. Elaine edged towards the coast of Guangdong after moving across Taiwan, necessitating the hoisting of Standby Signal No. 1 in Hong Kong. On the morning of 1 September 1985, Odessa and Pat were over the Sea of Japan, Ruby east of Japan, Tess east of the Philippines, and Skip near the International Date Line. Tess eventually entered the South China Sea and intensified into a typhoon, passing within 200 km southwest of Hong Kong and necessitating the issuance of the Gale or Storm Signal No. 8.

Figure 1

Figure 1      Tracks of Bess, Carmen, Della, Elaine and Faye in 1960

Figure 2

Figure 2      Tracks of Odessa, Pat, Ruby, Skip and Tess in 1985

Given that tropical cyclones typically require a separation spacing of 10 - 15 degrees to develop, and in terms of longitude the basin of western North Pacific and South China Sea has a span of about 80 degrees from west to east (100oE to 180o), there is probably a reason why the maximum number of co-existing tropical cyclones so far is five. Although six is not impossible, but we will have to wait and see.

C.W. Choy and M.C. Wu

  • The unsettled weather

  • Friday, 9th September 2016

In the weather report issued last weekend, this week's weather was expected to be unsettled. Do you know what "unsettled" means here?

"Unsettled" actually means that the weather can be rather changeable or can vary a lot. It may not be rainy all day long, but under suitable conditions, rain or even thunderstorms can occur. In meteorology, there is a term known as atmospheric instability. One of the ways weather forecasters analyse instability is to study the vertical variation of temperatures and the humidity of the atmosphere, which are commonly represented by the well-known K-index. It can be calculated based on upper-air measurement data, and numerical weather prediction model output from computer can also provide forecasters with the predicted values for the next few days. The K-index consists of three parts: (1) temperature lapse rate, which is the extent of drop in temperature with height; (2) moisture content at lower atmosphere; and (3) the degree of moisture saturation at the middle atmosphere. Generally speaking, K-index is higher for larger temperature lapse rate (i.e. higher temperature near surface and lower temperature aloft), higher moisture content at lower atmosphere, and closer to saturation of moisture at middle atmosphere. A high K-index means that convective activity can easily be triggered, resulting in precipitation or thunderstorms.

The Observatory operates an Automatic Upper-air Sounding System and launches sounding balloons twice a day to obtain upper-air meteorological observation. Take the sounding data in the morning of 5 September this year as an example (Figure 1), the near surface temperature was high and the temperatures aloft dropped rapidly with height (red circles in Figure 1). The warm air near the surface rose easily due to its lower density, resulting in convective activity. Besides, the sounding data showed a small difference between air temperatures and dew point temperatures, which meant that the humidity of the atmosphere was high and there was abundant moisture content for producing precipitation. In general, thunderstorms can be triggered when K-index reaches 33, and the index calculated from this set of sounding data was 37. In fact, there were showers and thunderstorms in Hong Kong on that day, bringing 30 millimetres of rainfall generally over the territory. Rainfall even exceeded 70 millimetres over parts of the territory (Figure 2).

Figure 1

Figure 1      The upper-air sounding profile in the morning of 5 September 2016. Small difference between air temperatures and dew point temperatures indicates higher humidity and near saturation in the atmosphere.

Figure 2

Figure 2      Distribution of total rainfall on 5 September 2016.

Although the atmosphere is unstable, we still need triggering mechanism to initiate precipitation and thunderstorms. This includes, for instance, a trough of low pressure or an upper-air disturbance near Hong Kong, which can enhance the uplift of air. There are other factors like mesoscale or small-scale weather systems and orographic effect, which are also considered and analysed by weather forecasters. From now on, when you learn from the Observatory that unsettled weather is expected in the next few days, remember to stay tuned to our more updated and detailed weather assessment!

Stephen P.W. Lau     L.S. Lee

  • Story of a Bolt

  • Tuesday, 6th September 2016

What is meant by "the height" of sea level in the Observatory's reports of astronomical high tides or the highest sea level during the passage of a typhoon?

The following example is extracted from a tropical cyclone bulletin issued by the Observatory during the passage of Typhoon Kalmaegi on 16 September 2014: "A high water level of 3 metres (m) above Chart Datum is expected at Quarry Bay between 1:00 a.m. and 4:00 a.m. The high water level may cause flooding in low-lying areas."

Chart Datum (CD) is not only the reference often quoted for describing sea level, but also the reference quoted for describing sea depth[1]. For such purposes, it is therefore desirable that the reference is set at a level low enough to remain under the sea water most of the time, and as such is usually defined in terms of the lowest astronomical tides. As the tide levels vary from place to place, the levels of Chart Datum for different places are not the same. In Hong Kong, the definition of Chart Datum all started with a bolt (Figure 1).

Figure 1

Figure 1      The 'Rifleman's Bolt' (left) and details of the soundings taken by H.M. Surveying Vessel Rifleman in 1866 (right) as displayed at the Hong Kong Maritime Museum, on loan from the Lands Department (courtesy of the Lands Department and the Hong Kong Maritime Museum).

Back in 1866, the surveyors of the H.M. Surveying Vessel Rifleman were mapping the northern shore of Hong Kong Island and fixed a bolt (then known as the 'Rifleman's Bolt') on a block of granite cornerstone at the Royal Naval Dockyard (now near the MTR Admiralty Station, Wanchai) as a benchmark. The level of the bolt was found to be 17 feet 10 inches (5.435 m) above a datum measured at the time. This datum was subsequently adopted as the Principal Datum (PD), serving as a reference for all heights and levels measured on land[1]. The height above Principal Datum is obtained by adding 5.435 m to the height as measured against the Rifleman's Bolt. Making reference to the Rifleman's Bolt, more than 1,700 (up to January 2016) benchmarks had been established in the territory since then for height surveying purpose [2].

Dr William Doberck, the first Director of the Hong Kong Observatory, conducted tidal measurements in Victoria Harbour near Tsim Sha Tsui using a tide gauge in 1887-1889 and derived the mean sea level, the highest and lowest tide[3]. The tidal readings were subsequently analysed by the then Nautical Almanac Office of UK and the analysis results formed the basis for tide predictions for Hong Kong published in the tide tables by the Admiralty, UK since 1893[4]. In the early 1910s, a datum of 19 feet 2 inches (5.842 m) below the Rifleman's Bolt, corresponding to an exceptionally low spring tide (Figure 2) was adopted for the daily tide predictions in the tide tables. In 1917, the zero of the tidal predictions for Hong Kong in the tide table was changed to 18.31 feet (5.581 m) below the Rifleman's Bolt or 0.48 foot (0.146 m) below the Principal Datum (Figure 3), reference for which had been made to the lowest tide level during Spring Water in India(1). Since then, it was also adopted as the datum of the Admiralty charts (called as Chart Datum later) of Hong Kong for all offshore bathymetrical surveys[6,7]. Figure 4 illustrates the relationships between the Rifleman's Bolt and the mean sea level, tide level, sea depth and various datums.

Figure 2

Figure 2      Description of datum for tide for Hong Kong in "Tide Tables for Standard Ports in the United Kingdom and Other Parts of The World for the Year 1914" published by the Admiralty, UK (courtesy of the UK Hydrographic Office (

Figure 3

Figure 3      Description of datum for tide for Hong Kong in "Tide Tables for Standard Ports in the United Kingdom and Other Parts of The World for the Year 1917" published by the Admiralty, UK (courtesy of the UK Hydrographic Office (

Figure 4

Figure 4      Schematic showing the relationships between the Rifleman's Bolt and the mean sea level, tide level, chart depth and various datums (not to scale), with the variation of tides shown in blue.

The Rifleman's Bolt was temporarily re-located in 1959, and then moved to the east wall of Blake Block in the HMS Tamar buildings in 1962 after demolition of the original site. With the British naval base moving to the Stonecutters Island in April 1997, the Rifleman's Bolt was also removed and taken away. It was later returned to the Lands Department of Hong Kong for retention[8]. With the completion of its historical mission, the Rifleman's Bolt is now displayed as a piece of monument at the Hong Kong Maritime Museum.

H.Y. Mok and W.H. Lui


(1) A tidal low water datum, designed for regions of mixed tides, originally developed for parts of the Indian Ocean[5].


[1] Website of Lands Department on Explanation of Levels in Hong Kong (

[2] Information on benchmarks in Hong Kong:

[3] Observations made at the Hong Kong Observatory in the year 1889, W. Doberck, Director, 1891

[4] Report of the Director of the Observatory for 1891 and 1892

[5] Glossary of Meteorology (

[6] High-level coastal deposits in Hong Kong, GEO Report No. 243, J.C.F. Wong and R. Shaw, Geotechnical Engineering and Development Department (

[7] Explanatory Notes on Geodatic Datums in Hong Kong, Survey & Mapping Office, Lands Department, page A5 (

[8] Hong Kong Maritime Museum Exhibit on 'Rifleman's Bolt and brass plaque for commemorating the relocation of the Bolt in 1962'